Closures

function addTo(passed) {
  const add = function (inner) { // creating closure
    return passed + inner; //injecting var 'passed' in closure
  };

  return add;
}

var addThree = addTo(3);
var addOne = addTo(1);
var addFour = addTo(4);

console.log(addThree(3));
console.log(addOne(2));
console.dir(addFour);